1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly, to a semiconductor device having a voltage sensing element for sensing an output voltage.
2. Description of the Background Art
Conventionally, a voltage sensing element for monitoring an output voltage is provided, for example, in a circuit for driving a fluorescent lamp. FIG. 19 is an equivalent circuit diagram showing a conventional circuit for driving a fluorescent lamp. Referring to FIG. 19, in a conventional circuit for driving a fluorescent lamp, a power device bridge including insulated gate bipolar transistors (IGBT) 101 and 102 is connected to a power source V.sub.cc. A fluorescent lamp is represented by capacitance C and connected between insulated gate bipolar transistors 101 and 102. Insulated gate bipolar transistor 101 has a gate connected to a control circuit 106 in an IC 104. Insulated gate bipolar transistor 102 has a gate connected to a control circuit 107 in an IC 105.
A voltage dividing resistor circuit (a voltage sensing element) consisting of resistors 110a and 110b is connected between insulated gate bipolar transistors 101 and 102. One end of resistor 110a is connected to control circuit 106. The connecting portion of resistors 110a and 110b is connected to control circuit 107.
In a conventional circuit for driving fluorescent lamp, it is necessary to monitor an output voltage V1 to control the same. As output voltage V1 can rise as high as approximately 200 V, the output voltage cannot be directly input to control circuits 106 and 107. Accordingly, a conventional circuit for driving fluorescent lamp requires a voltage sensing element for inputting an appropriate low voltage to control circuits 106 and 107. A voltage dividing resistor circuit (110a, 110b) is conventionally used as such a voltage sensing element.
However, a conventional circuit for driving a fluorescent lamp has a problem that resistors 110a and 110b consume much electric power because of high voltage applied to resistors 110a and 110b constituting the voltage dividing resistor circuit. Furthermore, when output voltage V1 is low, the voltage divided in proportion to resistance will be quite small. Therefore, when output voltage V1 is low, output voltage V1 cannot be measured very accurately by the conventional voltage dividing resistor circuit.